Duloxetine hydrochloride (duloxetine HCl) is a dual reuptake inhibitor of the neurotransmitters serotonin and norepinephrine. It is used for the treatment of stress urinary incontinence (SUI), depression, and pain management. Duloxetine hydrochloride is known by the chemical name (S)-(+)-N-methyl-3-(1-naphthalenyloxy)-3-(2-thienyl) propanamine hydrochloric acid salt, and has the following structure.

Duloxetine, as well as processes for its preparation, is disclosed in U.S. Pat. No. 5,023,269. EP Patent No. 457559 and U.S. Pat. Nos. 5,491,243 and 6,541,668 also provide synthetic routes for the preparation of duloxetine. U.S. Pat. No. 5,023,269 discloses preparing duloxetine by reacting (S)-(-)-N,N-Dimethyl-3-(2-thienyl)-3-hydroxypropanamine with fluoronaphtalene (Stage a), followed by demethylation with phenyl chloroformate or trichloroethyl chloroformate (Stage b) and basic hydrolysis (Stage c), according the following scheme.
The conversion of duloxetine to its hydrochloride salt in ethyl acetate (Stage d) is described in U.S. Pat. No. 5,491,243 and in Wheeler, W. J., et al, J. Label. Cpds. Radiopharm, 1995, 36, 312.
As illustrated in the above scheme, DNT is an intermediate in the preparation of duloxetine. DNT has an N,N-dimethyl group instead of a secondary amine.
U.S. Pat. No. 5,023,269 describes the preparation of DNT-oxalate from DNT. See Example 1.
The oxalate salt of U.S. Pat. No. 5,023,269 is problematic for use on an industrial process. Oxalic acid has to be used to prepare the oxalate. Oxalic acid is highly toxic. Therefore, there is a need in the art to prepare duloxetine HCl with a relative high purity with a process that is suitable for industrial scale.
Stereochemical purity is of importance in the field of pharmaceuticals, where many of the most prescribed drugs exhibit chirality, and the two isomers exhibit different potency. Furthermore, optical purity is important since certain isomers may actually be deleterious rather than simply inert. Therefore, there is a need to obtain the desired enantiomer of duloxetine HCl in high enantiomeric purity.
A composition of DNT is often contaminated with enantiomeric impurity. This enantiomeric impurity generally carries over to the final pharmaceutical product, i.e., duloxetine HCl. The present Applicants have found out that formation of the oxalate salt as carried out in EP Patent No. 457559 does not reduce the amount of the enantiomeric impurity (enantiomer R). There is a need in the art for a process that reduces the quantity of enantiomer R present in DNT.